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1. Field of Invention
This invention relates generally to electromechanical actuators of the type having an electric motor driving a rotary output shaft through a torque amplifying gear train.
More specifically, the invention relates to an electromechanical actuator with a brushless motor and contactless angular position sensors that provide both motor commutation signals and output shaft angular position signals to achieve a high-reliability, precision actuator.
2. Background Art
Electromechanical actuators have historically utilized AC or brush DC motors with potentiometers for feedback. Brushes in the motors and wipers in the potentiometers have led to limited life and low reliability for these types of actuators. More recently, the trend in precision electromechanical actuators is to utilize a brushless DC motor with a resolver, optical encoder, or switching Hall-effect device for motor commutation, a gear reducer for torque amplification, and a resolver, optical encoder, or rotary-variable-differential-transformer (RVDT) for sensing the angular position of the output shaft. These output shaft position feedback sensors are typically self-contained units driven by gears off the actuator output shaft. They are also substantially more expensive than conventional potentiometers, often requiring AC excitation and demodulation electronics to obtain useable output signals, and/or are unreliable in low temperature, moist environments. Consequently, precision actuators utilizing these types of sensors are generally more complicated and more expensive than actuators with more conventional potentiometer feedback.
Recent efforts to achieve lower-cost, yet reliable and accurate electromechanical actuators have included use of integrated contactless magnetic field sensor elements such as Hall-effect devices or magnetoresistive (MR) sensors. These sensor elements are relatively low cost, and are capable of generating electrical output signals when exposed to a rotating magnetic field. Hall-effect sensors utilize a current-carrying semi-conductor membrane to generate a low voltage perpendicular to the direction of current flow when subjected to a magnetic field normal to the surface of the membrane. Magnetoresistive sensors utilize an element whose resistance changes in the presence of a changing external magnetic field.
One group of prior electromechanical actuators utilize integrated Hall-effect sensors to provide signals that are digital in nature, generating pulses as a function of shaft rotation, or discrete signals for incremental shaft angles. These digital signals are generally developed by sensing the passage of notches, magnets, saturating magnet poles, or other discrete signal generating arrangements on a rotating shaft, and are used for motor commutation and/or actuator output shaft position sensing in the actuator. For example, Takeda et al., U.S. Pat. No. 5,422,551 uses Hall-effect sensors to generate pulse signals for motor control in a power window drive mechanism. Collier-Hallman et al., U.S. Pat. No. 6,002,226 uses Hall-effect sensors to generate pulse signals for motor control in an electric power steering system. Integrated Hall-effect sensors generating digital control signals are also shown in the motor controls of Coles et al., U.S. Pat. Nos. 6,104,152 and 6,124,688; Redelberger, U.S. Pat. No. 6,091,220; and Hans et al., U.S. Pat. No. 5,598,073. In Ritmanich et al., U.S. Pat. No. 6,198,243, integrated Hall-effect devices generate a pulsed output from rotation of an actuator output shaft for stepper motor control. As noted above, actuator and motor controls utilizing integrated magnetic field sensors as digital signal generators often require pulse-width modulation, or are otherwise relatively complicated to obtain, process and utilize the digital output signals from the sensors. And the accuracy of such devices is limited by the number of pulses per revolution developed from the sensed rotating element.
Another group of prior electromechanical actuators utilize integrated Hall-effect devices to produce analog signals indicative of the angular position of the output shaft for closed-loop control of the actuator. Electromechanical actuators of this type are shown in Peter et al., U.S. Pat. No. 5,545,961, Weiss et al., U.S. Pat. No. 6,097,123, and Fukumoto et al., U.S. Pat. No. 6,408,573. In general, these include annular magnets provided with sets of alternating N-pole/S-pole combinations coupled to the rotary output elements of the actuator, and Hall-effect sensors arranged around the magnet to produce analog output signals that are processed to obtain the angular position of the output element. Although capable of sensing angular position through 360 degrees of rotation, the accuracy of these types of actuators is limited to the accuracy of the Hall-effect sensing elements, which is currently, typically in the neighborhood of xc2x12 degrees, without provisions for special magnet magnetization processes, special sensor configurations, temperature compensation or reference calibration.
To advance the electromechanical arts, and to address the above-identified drawbacks of prior actuators of the same general type, there is a need for an improved electromechanical actuator that is capable of accurately controlling the angular position of a rotary output shaft, with the high reliability and long life available with the use of a brushless motor and contactless sensors, but without the high cost and complexity associated with use of resolver, encoders, or RVDTs. There is also a need for an improved accurate, high-reliability actuator that can be economically manufactured and compactly packaged.
For detailed discussion of position sensor configurations utilizing such magnetic field sensor elements, reference is made to Frederick et al, U.S. patent application Ser. No. 10/087,322, filed Feb. 28, 2002, and Seger et al, U.S. patent application Ser. No. 10/367,459, filed Feb. 14, 2003, both of which are assigned to the assignee of the present invention, and the discussions of which are incorporated herein by reference.
An important objective of the present invention is to provide an improved electromechanical actuator which can precisely control the angular position of an output shaft, but which is economical to manufacture.
Another important objective of the invention is to provide an actuator without motor and sensor contacts, brushes and wipers to improve actuator life and reliability as compared with many prior economical actuators of the same general type.
Another important objective of the invention is to provide an actuator that accurately computes and controls the position of the output shaft with enhanced accuracy without the high cost and complexity associated with use of resolver, encoders, RVDTs and like sensor components of many prior precision actuators.
Another important objective of the invention is to provide the foregoing high-reliability, accurate actuator in a compact package utilizing economical, standard components.
A detailed objective is to achieve the foregoing by providing an electromechanical actuator with high-reliability contactless brushless motor and contactless angular position sensing elements comprising simple magnets and magnetic field sensing elements to produce both motor commutation signals and shaft position signals.
Another detailed object is to achieve a compact actuator design by integrating the functional motor and sensor components around a common axis of rotation.
Another detailed objective is to use both the motor commutation signals and output shaft position signals in a unique algorithm to achieve enhanced precision control of the angular position of the output shaft.
These and other objectives and advantages of the invention will become apparent from the following detailed description when taken in conjunction with the accompanying drawings.
The objectives of the invention are accomplished in one preferred embodiment actuator with a brushless electric motor, an integrated motor commutation sensor comprising an annular two-pole magnet connected to rotate with the motor shaft and a pair of ratiometric Hall-effect devices for sensing the angular position of the magnet, a step-down geartrain coupled between the motor shaft and an output shaft (i.e., a rotatable output element), an integrated output shaft position sensor comprising a second annular two-pole magnet connected to rotate with the output shaft and a second pair of ratiometric Hall-effect devices for sensing the angular position of the output shaft magnet, and a digital-signal processor-based sensor computation and motor control circuit. The Hall-effect devices sense the magnetic field of each magnet as it rotates and provide output signals indicative of the angular position of the magnet over a full 360 degrees of rotation. A controller module computes the precise angle of the output shaft from the sensed positions of the magnets, compares the computed output shaft angle with an input position command, and provides logic signals to a motor power controller module to energize the appropriate motor windings and turn the motor in the direction necessary to drive the output shaft towards the commanded position.
As in any closed-loop control system, the accuracy of the actuator is primarily dependent upon the accuracy of the output shaft position sensing system. In the present invention, a highly accurate position sensing system is implemented economically and compactly by adding a pair of magnets and associated magnetic field sensors, wherein one magnet is connected to the motor shaft, the second magnet is connected to the output shaft, and rotation of the two magnets is coupled by the step-down ratio of the actuator geartrain such that the motor shaft rotates multiple revolutions for one turn of the output shaft.
With this arrangement, the output shaft magnet is used to generate signals to calculate a coarse indication of output shaft angle. In other words, the sensed angle of the output shaft magnet, as calculated by the digital signal processor, provides an indication of output shaft angle within the sensing accuracy of the magnet and magnetic field sensors. Current state-of-the-art in standard magnets and solid-state flux sensors can typically provide an indication of shaft angle within xc2x12 degrees over 360 degrees of rotation and xe2x88x9254 to 125xc2x0 C. of temperature variation without special magnetization or sensor configurations, electronic temperature compensation, or reference calibration data. Since the angular rotation of the motor shaft magnet can be sensed with the same degree of accuracy, and its rotational angle is a fixed multiple of the angular rotation of the output shaft, the sensed position of the motor shaft magnet can be used to obtain a more accurate indication of the output shaft angle with an improvement in accuracy approximately proportional to the interconnecting gear ratio.
To compute the precise angular position of the output shaft, i.e., to compute the angular position of the output shaft with the improved accuracy, the sensed angular position of the output shaft magnet is used to provide an absolute measure of the output shaft position at all motor shaft angles, and to predict the angle of the motor shaft as calculated by multiplying the sensed angle of the output shaft magnet by the gear ratio. The difference in the calculated angles of the two magnets is then divided by the gear ratio to obtain a correction factor that is applied to the sensed angle of the output shaft to compute a more precise output shaft position. Alternately, the angle of the output shaft magnet is utilized to count the number of complete turns of the motor shaft magnet, the result of which is added to the sensed angle of the motor shaft magnet. This total motor shaft rotation is divided by the gear ratio to provide an accurate measure of output shaft angle. Thus, the position of the output shaft is accurately computed as a function of the sensed positions of both the output shaft magnet, the motor shaft magnet, and the gear ratio connecting the two magnets. In implementing this aspect of the invention, the gear ratio between the magnets must be less than 360 degrees divided by the maximum position sensing error of the output shaft magnet to accurately predict the number of revolutions the motor shaft has traversed. Alternately stated, the step-down ratio must be less than the inverse of the accuracy in parts per hundred for which the rotational angle of the output shaft magnet can be sensed.
In the preferred embodiment actuator, a circular or annular magnet is fixed to or around the motor shaft. This configuration allows the shaft, or an extension therefrom, to extend through the center of the magnet for ease of attachment, and for compact packaging of the magnet in the actuator. The preferred magnet, made from ALNICO or samarium cobalt material or comparable material chosen for thermal stability, is magnetized with two poles 180 degrees apart on its outer radial surface or end face to establish a uniformly and periodically varying magnetic field as the magnet is rotated. The magnetic field sensors are positioned proximate the magnet to sense different non-saturating components of the varying magnetic field as the magnet rotates, and provide periodic phase-shifted or differential voltage signals indicative of and preferably proportional to the strength of the magnetic field sensed.
The Hall-effect devices are preferably angularly spaced 90 degrees from each other around the magnet, or otherwise positioned at operative right angles to each other, to sense orthogonal components of the periodically varying magnetic field, and to provide generally sinusoidal output voltage signals that are phase shifted 90 degrees from one another. This enables calculation of the sensed angular position of the magnet over 360 degrees rotation from the arctangent function of the ratio of the output signals from the sensors. These calculations are accomplished by the processor executing an arctangent function, or a single tracking observer or other algorithm commonly used to calculate a non-ambiguous angle from sine or cosine type signals. Alternately, for example, the magnetic field sensors may comprise a pair of magnetoresistive sensor elements having their magnetically sensitive axes at right angles to each other to develop periodically varying differential voltages indicative of and from which the angular position of the magnet is calculated.
The output shaft is provided with a second, similar annular magnet and associated magnetic field sensor set through the step-down geartrain which causes the output shaft magnet to rotate through one revolution as the motor shaft is turned through multiple revolutions. In the preferred embodiment, this is accomplished with a multiple-stage simple planetary geartrain with a typical overall step-down ratio of between 20-to-1 and 80-to-1. The step-down ratio is preferably an integer to maintain a direct relationship between the position of the output shaft sensor and the motor shaft sensor. If the step-down ratio is a non-integer, the output shaft rotation would normally be limited to not exceed 360 degrees to avoid the need to establish a datum or zero point with regard to the output shaft and motor shaft sensors. The multiple-stage planetary geartrain allows for an extremely compact actuator by enabling location of the output shaft magnet near the motor and controller electronics at the end of the actuator opposite the coupling between the output shaft and the load, and securing the output shaft magnet to a rod that passes through the center of the motor and geartrain and is secured at its other end to the output shaft.
The analog voltage signals from the magnetic field sensors are converted to digital format and utilized by the processor to calculate the sensed rotational angles of the individual magnets, and compute the precise angular position of the output shaft. As noted above, the processor also provides or uses the computed output shaft angle as an actuator position feedback signal to establish logic signals for control of the current in the motor windings and closed-loop position control of the actuator output shaft in response to the input command signal. If the input command includes target output shaft rotational time derivative parameters (i.e., rotational speed and/or acceleration information), the processor is further configured to calculate the applicable time derivatives from the rate of change of the sensed positions of the magnets, precisely compute the associated time derivative of the output shaft angle as a function thereof and the step-down gear ratio of the geartrain, compare the computed output shaft time derivative with the input command, and provide control signals to the motor control module for commutating the motor and controlling the subject time derivative of the output shaft. Overall, this arrangement uses simple calculations based on the sensed angular positions of the magnets and the gear ratio to precisely control the output shaft, and allows the sensing elements to be conveniently and compactly packaged around a central axis of the actuator.